Processes for making alpha alumina are known in the prior art. Alpha alumina is usually prepared by calcining aluminum hydroxides or transition phase aluminas or other inorganic or organic compounds of aluminum at a maximum temperature of 1200.degree. C. or more. At approximately 1000.degree. C. a slow transformation starts, accelerating with increasing temperature. The temperature and time of calcination are selected to obtain complete conversion to alpha alumina and to eliminate transition forms of alumina. The calcined powder is then often milled to a fine particle size, generally less than about 10 microns. Sintered ceramic articles can then be made by pressing or slip casting the ground alpha alumina powder to a desired shape and then firing the "green" shape to a temperature in the range of about 1400.degree.-1800.degree. C. to obtain the final product.
Kuklina U.S. Pat. No. 3,950,507 and Weber U.S. Pat. No. 4,379,134 are prior art patents disclosing processes for making alpha alumina particles. The products have good purity. However, the processes disclosed in these patents include calcination temperatures of about 1200.degree. C. or more. Consequently, reduction of the resulting products to particles having an average size of less than about 10 microns requires considerable time and energy.
The exact mechanism of mineralization is not clear, but somehow a mineralizer acts to facilitate transfer of material from high energy to low energy sites. Crystal growth during calcination is affected by temperature, type, and amount of mineralizer present (probably on the alumina surface) and condition (amount of alpha present) of the alumina at the time the mineralization reaction is initiated. If the alumina has been pre-calcined to pure alpha phase, limited growth (during subsequent nineralization) occurs, probably by a process analogous to ripening in precipitation where a material from smaller crystals migrates to larger crystals to mninimize excess surface area. In the usual industrial case, alumina is heated rather slowly in a rotary kiln and some conversion to alpha has already occurred (beginning about 1000.degree. C.) before the mineralization reaction is initiated (at about 1200.degree. C. with aluminum fluoride). It is believed that the mineralization with aluminum fluoride is dependent on the hydioylsis of aluminum fluoride which does not occur in any appreciable amount until about 1150.degree. C. At this point, transition alumina (high surface, unstable structure) is rapidly depleted as material is transported to alpha alumina crystal sites causing rapid crystal growth. The reaction is autocatalytic because loss of surface energy results in a significant rapid increase in temperature and desorption of fluoride which increases vapor phase fluoride content in the immediate vicinity of the reaction. Both of these effects accelerate the reaction. The resulting alpha alumina crystal size is mostly affected by the number of alpha sites present at the initiation of mineralization (few sites resulting in large crystals, many sites resulting in small crystals). If the mineralization temperature is kept low, fewer alpha sites are produced from the natural unmineralized reaction and larger crystals can be produced. Once the alpha transformation is completed, further crystal growth Occurs by the much slower ripening mechanism described above. The role of soda in calcination of alumina deserves special attention because it is nearly always present in commercial aluminas. It is commonly known that increasing soda content retards alpha alumina transformation in normal calcinations without mineralizers. Fluoride has been shown to mineralize crystal growth in alumina with very little soda present but fluoride in the presence of soda results in larger crystals. This strongly implies that sodium does play a key role in the mineralization process. If sodium fluoride is present (generally greater than 1 wt. %) beta alumina can form which tends to have very large, plate-like crystals with high aspect ratios.
For mineralization to be effective, it is necessary to have a high enough temperature to initiate the reaction and enough mineralizer present to facilitate transport. These two conditions can work against each other if the mineralizer has a high degree of volatility. For crystal growth to be controllable, the alumina must be in the proper state of preconditioning when the mineralization reaction is initiated. Our invention allows the reaction to occur at 950.degree.-1025.degree. C. because we are able to bring sufficient amounts of necessary mineralization components (sodium, alumina and fluoride) intimately together. This can be accomplished by wet loading the fluoride onto the alumina before calcination or bringing fluoride containing vapor into intimate contact with the other two components at approximately 1000.degree. C. The process uses the inherent soda in the alumina and adds no alkali to the process. Our liquid load technique actually substantially reduces the alkali content of the alumina at ambient temperature (before mineralization).
It is known from MacZura et al U.S. Pat. No. 3,655,339 that the relative proportions of soluble and insoluble soda in calcined alumina are dependent upon total soda content. Using well washed hydrate feedstocks with varying soda contents, the patentees found that 92.7% of all soda could be leached from calcined alumina containing 0.16 wt. % soda, while only 36.4% of all soda was leached from calcined alumina containing 0.55 wt. % soda. There was a steady decline in percentage reduction of soda content in 8 test samples as the initial total soda content increased.
Normally in fluoride mineralized aluminas the proportion of soluble soda is lower after calcination, possibly because the soda is tied up with beta alumina.
The liquid load/leach process of the present invention, followed by low temperature calcination, enables removal of 88-98% of total soda content from the alumina. Soda content is reduced by 78% with the fluoroform calcination process of the present invention.
A principal objective of the present invention is to provide an efficient and economical process for making alpha alumina particles at reduced temperatures.
A further objective of our invention is to provide a process for making alpha alumina particles having reduced sodium content.
A related objective of the invention is to provide a process for making alpha alumina particles of reduced particle size wherein less time and energy are required for grinding than in the prior art.
Additional objectives and advantages of our invention will become apparent to persons skilled in the alt from the following detailed description of a preferred embodiment.